Electric vehicles, hybrid electric vehicles (HEV's), and indeed any vehicle that utilizes an electric machine, such as an electric motor, may be configured to use the electric machine to provide regenerative braking to at least assist in stopping the vehicle. In addition, non-electric vehicles may also be configured to provide regenerative braking, for example, through the use of a hydraulic system. Regenerative braking provides a number of advantages over using a friction braking system exclusively. For example, the use of regenerative braking, whereby an electric motor provides negative torque to the vehicle wheels, reduces wear on the friction elements of the friction braking system. In addition, during regenerative braking, the motor can function as a generator, producing electricity that can be used immediately, or stored in a storage device, such as a battery.
Because of the advantages associated with regenerative braking, current regenerative braking control systems attempt to apply the maximum regenerative braking torque so that overall vehicle efficiencies are maximized. It may be desirable to avoid this strategy, however, when the vehicle, and thus the motor, is operating at a very low speed. This is because although the motor is capable of producing very high torque at low speeds, it does so with poor efficiency. Therefore, the regenerative braking torque may be controlled to gradually reduce to zero at low vehicle speeds.
One example of a control method for a regenerative braking system is described in U.S. Pat. No. 6,508,523 issued to Yoshino on Jan. 21, 2003. The Yoshino control method controls the regenerative braking torque to emulate the braking torque of an internal combustion engine. In particular, the value of the regenerative braking torque supplied by an electric motor is almost constant when the vehicle speed is above some predetermined speed. When the vehicle speed drops below this predetermined point, the regenerative braking torque is controlled to gradually go to zero in a way that best emulates the braking torque of an internal combustion engine.
One limitation of the control system described in Yoshino is that it does not address the need to change the set point at which the regenerative braking torque begins to be reduced to zero. For example, as described above, the braking torque may be gradually blended out from some maximum available value to zero when the vehicle speed is low. There are times, however, when this strategy may result in undesirable vehicle behavior. One of these times is during heavy vehicle decelerations, where the blending out of the regenerative braking torque may become abrupt and rough. In such cases, it may be desirable to adjust the point at which the blending out of the regenerative braking torque begins. In addition, when regenerative braking is used in conjunction with friction braking, there may be times when the friction braking system experiences some reduced braking capability. In these cases, it may be desirable to begin the blending out of the regenerative braking torque at a point that is different than during a heavy deceleration.
Therefore, a need exists for a method for controlling regenerative braking in a vehicle which not only blends the regenerative braking torque from some operating level down to zero when the vehicle speed is low, but also uses different points to begin the blending out, thereby accommodating such situations as various levels of high deceleration and reduced friction braking capability.